The present invention relates, in general, to the field of direct current (xe2x80x9cDCxe2x80x9d) motor control circuits. More particularly, the present invention relates to an integrated circuit (xe2x80x9cICxe2x80x9d) xe2x80x9cHxe2x80x9d switch, or H-bridge, motor control circuit incorporating a boost capacitor for the low-side metal oxide semiconductor (xe2x80x9cMOSxe2x80x9d) transistor switching devices in the H-bridge in which the characteristics of the respective capacitors are closely matched to that of the associated switching device.
The DC low voltage fractional horsepower motor is a standard for applications that require bidirectional shaft control such as cassette tape drives, automobile power windows, seats, door locks or windshield wipers, robotic mechanical controls, small industrial machines etc. As such, H-bridge circuits have been developed in lieu of mechanical reversing gears and linkages for reversing the motor supply voltage. In an H-bridge, opposing top and bottom switching devices (bipolar, Darlington or MOS transistors for example) couple the DC motor input leads to a voltage source and circuit ground to control the motor direction by switching the voltage polarity, which, in turn, switches the motor""s shaft rotation either clockwise or counter-clockwise.
Bipolar transistors are generally not satisfactory in these applications due to their relatively large base drive requirement and Darlington transistors exhibit a large forward voltage drop across the devices which reduce voltage to the motor. For these reasons, MOS transistors are often utilized in H-bridge applications as disclosed in U.S. Pat. No. 4,454,454 issued Jun. 12, 1984 for xe2x80x9cMOSFET xe2x80x9cHxe2x80x9d Switch for a DC Motorxe2x80x9d.
Still other H-bridge circuits have utilized MOS transistors in order to control the current through the load in addition to switching voltage polarity. In this regard, the low-side switching devices function as current generators to force a defined current through the load. However, after each transition, the current generators take a finite time to reach a steady-state condition. In order to reduce this time and speed the overall system operation, an additional charge, through the use of an inverter and associated boost capacitor, may be added to the gate of the low-side switching devices. However, for optimal performance, there must be a good match between the boost capacitors and the input capacitance of the switching transistors. This is especially difficult to achieve in integrated circuit H-bridges due to the inherent variations in processing technology.
As mentioned previously, in order to provide effective boost, the boost capacitors must be closely matched with the respective input capacitance of the associated switching transistors. In order to effectuate this effective matching despite manufacturing process variations, the present invention may utilize the source structure of the transistors for the formation of the respective boost capacitors. Since the capacitors are basically MOS devices themselves, the gate of the associated transistor forms one terminal of the capacitors while the P-well forms the other terminal. The gate of these capacitors may consist of more poly fingers extending from the respective transistors in order to reproduce the same border effect of these devices.
Particularly disclosed herein is an integrated H-bridge circuit including first and second pairs of high-side and low-side switching devices for selectively coupling an electrical load between first and second voltage supply lines. The circuit comprises a boost capacitor coupled to a switching terminal of each of the low-side switching devices, with the boost capacitor having a capacitance substantially equivalent to an input capacitance of an associated one of the low-side switching devices.
In a particular embodiment, disclosed herein is a boost capacitor for supplying a voltage potential to an associated MOS transistor. The boost capacitor comprises a capacitor body formed in a portion of a semiconductor substrate with the capacitor body being formed of a first impurity type. A plurality of regions are formed in the capacitor body, with the regions being formed of a second, opposite impurity type. An insulating layer overlies the capacitor body between the plurality of regions and a conductive layer overlies the insulating layer. The effective capacitance of the boost capacitor is substantially equal to the conductive layer-to-capacitor body and the conductive layer-to-plurality of regions capacitance and is substantially equal to an input capacitance of the associated MOS transistor.
Further disclosed herein is a method for forming an integrated H-bridge circuit including a boost capacitor and an associated MOS transistor. The method comprises the steps of: providing a semiconductor substrate and forming first and second wells in the substrate overlying first and second buried layers. A plurality of regions are diffused in the first and second wells of an impurity type opposite to that of the first and second wells and an insulating layer is formed overlying the first and second wells between the diffused regions. A conductive layer is also formed overlying the insulating layer. The various structures are formed concurrently of the same materials and are laid out such that the corresponding dimensions of each are substantially identical where possible.